The present invention relates to a radar apparatus.
In a radar apparatus, a target in a frequency bandwidth higher than Nyquist frequency which is a half the sampling frequency for AD conversion of a beat signal, may appear as a ghost by replicating to a frequency bandwidth lower than Nyquist frequency. Such a replication of a frequency is generated on a frequency spectrum, as a result of performing FFT analysis on the beat signal generated by mixing a reception wave and transmission wave, that is, a detection signal. FIG. 1 is a diagram illustrating a state where a frequency replicates in a frequency spectrum obtained by FFT analysis. In FIG. 1, the vertical axis indicates the strength of a reception signal and the horizontal axis indicates a frequency. In FIG. 1, two signal components TG1, TG2 are shown as the signal components of the target, in which TG1 indicated by a solid line is a correct signal component of the target and TG2 indicated by a dashed line shows a signal component generated by the replication due to Nyquist frequency.
When the replication of frequency is generated, which is shown in FIG. 1, it is difficult to accurately detect the distance to the target or the speed of the target. That is, TG2 which is actually a distant target is erroneously detected as a closer target.
In order to solve this problem, in the related art, for example, a baseband circuit has been provided with a band-pass filter having a precipitous cutoff characteristic. FIG. 2 is a diagram illustrating a state where the precipitous band-pass filter is set, in which the frequency characteristic of the band-pass filter is overlapped on the frequency spectrum obtained by the FFT analysis. In FIG. 2, the vertical axis indicates the magnitude of a reception signal and the horizontal axis indicates a frequency. In FIG. 2, two signal components TG1, TG2 are shown as the signal components of the target, in which TG1 indicated by a solid line is a correct signal component of the target and TG2 indicated by a dashed line shows a signal component generated due to the replication by Nyquist frequency. In FIG. 2, as the precipitous band-pass filter is set, the signal components of the target higher than Nyquist frequency are attenuated and the signal components generated by the replication are also attenuated. As a result, the signal components generated by the replication become less than a threshold, and thus an error in detection of TG2 is avoided. Patent Document 1 discloses a technology relating to the above. Patent Document 1 discloses a radar apparatus that determines incidence of the replication of the frequency.
Patent Document 1: JP-A-2004-069340
In the related art, as the cutoff characteristic of the band-pass filter is made precipitous, the number of parts of the circuit, such as a resistor, a capacitor, and an operational amplifier, in the band-pass filter may increase. As the number of parts increases, it is difficult to achieve a small-sized band-pass filter and the cost may increase. An available baseband bandwidth may be limited by the cutoff characteristic of the band-pass filter. To avoid these problems, for example, a superspeed AD converter may be used. The superspeed AD converter performs oversampling to form a digital filter by a signal processing, and thus the replication by Nyquist frequency can be removed. However, the cost of a radar apparatus may be considerably increased with the use of the superspeed AD converter.